SOUTHAMPTON OCEANOGRAPHY CENTRE CRUISE REPORT No. 24
RRS DISCOVERY CRUISE 233 23 APR - 01 JUN 1998
A Chemical and Hydrographic Atlantic Ocean Survey:
CHAOS Principal Scientist
D Smythe-Wright 1999
George Deacon Division for Ocean Processes Southampton Oceanography Centre
Empress Dock European Way Southampton S014 3ZH
UK
Tel: +44 (0)1703 596439 Fax: +44 (0)1703 596204
Email: D.Smythe-Wright@soc.soton.ac.uk
DOCUMENT DATA SHEET
AUTHOR: SMYTHE-WRIGHT, D et al PUBLICATION DATE: 1999
TITLE: RRS Discovery Cruise 233, 23 Apr-01 Jun 1998. A Chemical and Hydrographic Atlantic Ocean Survey:
CHAOS.
REFERENCE: Southampton Oceanography Centre Cruise Report, No.
24, 86pp.
ABSTRACT
RRS Discovery Cruise 233, CHAOS (Chemical and Hydrographic Atlantic Ocean Survey) combined a long meridional section notionally along 20°W from 20°N to Iceland with a detailed survey of the Rockall Trough. The meridional section was designed to i) establish the sources and sinks of halocarbons in subtropical and subpolar waters during spring bloom conditions; ii) to examine the decadal scale variability in the eastern Atlantic over the last 40 years by repeating the northern part of the WOCE A16 line first occupied in 1988 and again in 1993 (NATL 93), and parts of other sections occupied in 1957, 1973, 1983 and 1991; iii). to study the spreading mixing and ventilation rates of Labrador Sea Water, Mediterranean Water, and waters of Southern Ocean origin (Antarctic Intermediate Water and Antarctic Bottom Water) which extend into the northeast Atlantic. The detailed survey of the Rockall Trough comprised 4 zonal sections notionally at 57°N, 56°N, 54°N and 52°N in order to i) make a detailed study of the water masses in the Rockall Trough with particular emphasis on their circulation/recirculation patterns ii) to re-occupy stations along the Ellett line (57°N) to continue the time series dating from 1975. The sections were completed with CTD, LADCP, tracer chemistry (CFCs, nutrients, oxygen), alkalinity and pH measurements to full depth and a suite of halocarbon measurements together with sampling for plant pigments and biological species to 200m. Continuous measurements of atmospheric halocarbons,pCO2 meteorological measurements, VM -ADCP, depth, TSG, radiometer SST and navigation data were also made. All measurements were made to WOCE standards and the final data submitted to the WOCE programme.
KEYWORDS
ADCP, ALKALINITY, ATLNE, ATMOSPHERIC HALOCARBONS, BIOLOGY, CFC, CHAOS, C02, CRUISE 233 1998, DISCOVERY, HALOCARBONS.
ICELAND WATERS, LADCP, METEOROLOGICAL DATA, METEOROLOGICAL MEASUREMENTS, NORTHEAST ATLANTIC, NUTRIENTS, OXYGEN, pH, PLANT PIGMENTS, ROCKALL TROUGH, SISTeR, TRACER CHEMISTRY, TRACERS, WOCE
ISSUING ORGANISATION
Southampton Oceanography Centre Empress Dock
European Way
Southampton S014 3ZH UK
Copies of this report are available from:
National Oceanographic Library, SOC PRICE: £19.00
Tel: +44(0)01703 596116 Fax: +44(0)01703 596115 Email:no1@soc.soton.ac.uk SCIENTIFIC PERSONNEL- Leg I
Name Role Affiliation
Smythe-Wright, Denise Principal Scientist SOC-GDD Alderson, Steve CTD processing, LADCP (PI) SOC-JRD
Bonner, Rob Salts (PI), CTD technical SOC-GDD
Bryden, Harry CTD processing (PI) SOC-JRD
Davidson, Russell Pigments (PI), Species (PI) SOC-GDD
Day, Kate Oxygen University of Liverpool
Dimmer, Claudia Atmospheric gases (PI), Halocarbons University of Bristol Duncan, Paul Senior Computing Technician SOC-RVS
Hart, Virginie Nutrients (PI) SOC-GDD
Holliday, Penny CTD processing SOC-GDD
Jolly, Dave Instrumentation SOC-RVS
Jones, Gwyneth Data Processing SOC-ITG
Josey, Simon Meteorology SOC-JRD,
Laglera, Luis pC02 (PI), Alkalinity (PI) University of Las Palmas Pascal, Robin Meteorology, CTD technical SOC-OTD
Peckett, Cristina Halocarbons (PI) SOC-GDD
Poole, Tony Senior RVS Technician SOC-RVS
Redbourn, Lisa ADCP (PI), LADCP SOC-JRD
Roberts, Rhys Mechanical SOC-RVS
Rourke, Lizzy Oxygen (PI) SOC-JRD
Rymer, Chris Mechanical SOC-RVS
Schazmann, Ben Pigments, Species University of Galway Sheasby, Tom SST (PI), Oxygen University of Leicester
Short, John Computing SOC-RVS
Smithers, John CTD technical (PI) SOC-OTD
Soler-Aristegui, Iris pH (PI), Halocarbons SOC, University of Vigo Somoza-Rodriguez, Maria pC02, alkalinity, pH University of Las Palmas
Wilson, Chris Salts University of Liverpool
SCIENTIFIC PERSONNEL- Leg 2
Name Role Affiliation
Smythe-Wright, Denise Principal Scientist SOC-GDD Alderson, Steve CTD processing, ADCP (PI) SOC-JRD
Bonner, Rob Salts (PI), CTD technical SOC-GDD
Davidson, Russell Pigments (PI), Species (PI) SOC-GDD
Day, Kate Oxygen University of Liverpool
Dimmer, Claudia Atmospheric gases (PI), Halocarbons University of Bristol Duncan, Paul Senior Computing Technician SOC-RVS
Hart, Virginie Nutrients (PI) SOC-GDD
Jolly, Dave Instrumentation SOC-RVS
Jones, Gwyneth (TBC) Data Processing SOC-ITG
Josey, Simon Meteorology (PI) SOC-JRD
Laglera, Luis (TBC) pC02 (PI), Alkalinity (PI) University of Las Palmas
Meggan, Alex CTD, processing SOC-JRD
New Adrian XBT (PI), CTD processing SOC-JRD
Pascal, Robin Meteorology, CTD technical SOC-OTD
Peckett, Cristina Halocarbons (PI) SOC-GDD
Redbourn, Lisa LADCP (PI), ADCP SOC-JRD
Roberts, Rhys Mechanical SOC-RVS
Rourke, Lizzy Oxygen (PI) SOC-JRD
Rymer, Chris Senior RVS Technician SOC-RVS
Schazmann, Ben Pigments, Species University of Galway Sheasby, Tom SST (PI), Oxygen University of Leicester
Short, John Computing SOC-RVS
Smithers, John CTD technical (PI) SOC-OTD
Soler-Aristegui, Iris pH (PI), Halocarbons SOC, University of Vigo Somoza-Rodriguez, Maria pC02, alkalinity, pH University of Las Palmas
Wilson, Chris Salts University of Liverpool
SHIPS PERSONNEL
Name Rank
Avery, Keith Master Gauld, Phil Chief Officer Mackay, Alistair 2nd Officer Parrotte, Mark 3rd Officer Sudgen, Dave Radio Officer Moss, Sam Chief Engineer Clarke, John 2nd Engineer Crosbie, Jim 3rd Engineer Parker, Phil Electrician Drayton, Mick CPO (D) Lewis, Greg PO (D) Allison, Philip SIA Crabb, Gary SlA Kesby, Steve SIA Thomson, Ian SIA MacLean, Andy SlA Pringle, Keith SIA
Dane, Paul Senior Catering Officer Haughton, John Chef
Bryson, Keith Messman.
Osborn, Jeff Steward Mingay, Graham Steward
ACKNOWLEDGEMENTS
Firstly, I should like to thank the Master, Captain Keith Avery, for guiding me in my role as first-time Principal Scientist, his advise was much appreciated on many occasions. My sincere thanks also go to the officers and crew for their unending help throughout the cruise and, in particular, to the second officer, Alistair Mackay for his help with station timing/planning. Alistair's expertise in estimating, virtually to the half hour, where we would be in a week's time was unbelievable and without his help we would not have achieved so much.
I am most grateful to Melchor Gonzalez-Davila, University of Las Palmas and Aida Fernadez-Rios, University of Vigo for arranging equipment and scientific personnel for PC02, alkalinity and pH measurements. I am particularly thankful to Melchor for quickly arranging a replacement scientist when Stephen Boswell was unable to sail because of ill health. Without Melchor's quick response, the willingness of Maria Somoza-Rodriguez to join the ship in less than 12 hours, and the adaptability of Iris Soler-Aristegui to train Maria and thereby divide her time between pH and halocarbon analysis, the chemical results from the cruise would not have been so successful. I cannot over-express my gratitude to them.
I am also indebted to Sue Scowston, Andy Louch and Jackie Skelton of RVS operations and Rob Bonner for their handling of logistical arrangements; without Jacqui's help with travel many of us might never have reached Tenerife to join the ship.
My thanks are also given to the authorities of Mauritania, Algeria, Spain, Portugal, Ireland and Iceland for granting us permission to work in their territorial waters. So much more was achieved by having access to these waters.
Finally and, most importantly, I am extremely grateful to the entire scientific party for their dedication throughout a particularly long and arduous cruise. Without their assistance such a comprehensive data set would not have been collected;
everyone of them made my first experience as Principal Scientist an enjoyable one.
The cruise was funded by the UK Natural Environment Research Council, Southampton Oceanography Centre as a final contribution to the WOCE Hydrographic Programme and in
support of the SASHES (Sources and Sinks of Halogenated Environmental Substances) commissioned project.
Denise Smythe-Wright
Figure 1.1 CHAOS cruise track showing CTD stations positions. Julian day (1998) is given in normal text, station number in italics.
1 CRUISE DESCRIPTION 1. 1 Details
Cruise Name: Chemical and Hydrographic Atlantic Ocean Survey Designation: RRS Discovery Cruise 233
Port calls Tenerife to Farlie, Scotland with ship transfers in Vestmannaeyjar andThorlakshofn, Iceland
Cruise Dates: 23 April to 1 June 1998 WOCE designation: AR21
1.2 Outline and Objectives
CHAOS (Chemical and Hydrographic Atlantic Ocean Survey) combined a long meridional section along 20°W from 20°N to Iceland with a detailed survey of the Rockall Trough. It was a joint effort between the George Deacon (GDD) and James Rennell (JRD) Divisions of Southampton Oceanography Centre (SOC). It formed a fundamental part of the GDD study of the Sources and Sinks of Halogenated Environmental Substances and the JRD core programme Observing and Modeling the Seasonal to Decadal Changes in Ocean Circulation.
In addition, we were requested by the International WOCE community to complete the section to WOCE standards and submit the final data to the WOCE programme because the 20°W section was the only long meridional hydrographic section in the eastern North Atlantic during the late 1990s.
The objectives of the cruise were as follows
• to repeat a section, notionally along 20°W in the Northeast Atlantic, parts of which were occupied previously in 1957, 1973, 1983, 1988 and 1991, in order to examine the decadal scale variability in the eastern Atlantic over the last 40 years.
• to establish the sources and sinks of halocarbons in subtropical and subpolar waters during spring bloom conditions.
• to study the spreading, mixing and ventilation rates of Labrador Sea Water, Mediterranean Water, and waters of Southern Ocean origin (Antarctic Intermediate Water and Antarctic Bottom Water) which extend into the Northeast Atlantic.
• to make a detailed study of the water masses in the Rockall Trough with particular emphasis on their circulation/recirculation patterns.
• to contribute to the WOCE baseline survey of the North Atlantic.
1.3 Overview
The cruise commenced in Tenerife on 23 April with a 2.5 days passage leg to reach the start of the 20°W section at 20°N. During this time underway meteorological, atmospheric and hydrographic measurement were made and there was a test station at 26° 13.1' N, 17° 14.7' W in > 4000 m water when all bottles were fired at 3500 m.
We began the 20°W line in the early hours of Sunday 26 April with the first station (13415) at 20° 04.0' N, 20° 45.03' W. We then proceeded north-west to the 21° 20.0' W meridian working stations at 0.5 degree spacing (stations 13416- 13424). At 24°, 00.0' N we turned north and followed the 21° 20.0' W meridian to 35° 00.0' N (stations 13425-13466). From there, we made our way diagonally to 20' 00.0' W (stations 13466-13449) and continued due north from 36° 30.0' N.
The reason for the dog leg was (a) to avoid Mauritanian territorial waters; despite having clearance to work, we were unable to accommodate a Mauritanian observer due to pressure on berth space (b) to avoid a number of sea mounts in the region 23-27°N (c) to cross the top edge of the Maderia Abyssal plain and hence the deep flow as obliquely as possible. Between 36° 30.0' N and 52° 00.0' N we completed stations 13467-13480 and then turned east to occupy 8 stations along 52°N to the 500 in contour of the Porcupine Bank (stations 13481-13488).
We then made our way back to 20°W meridian and continued the 0.5 latitude spacing to 60° 00.0' N (stations 13489-13504). At this point it was necessary to make headway for Iceland to arrive in time for the ship's transfer next day. We completed the most northerly station of the section (station 13405) at 63° 19.3' N, 19° 59.3' W in the early hours of the morning of 22 May and steamed to the island of Vestmannaeyjar and then onto Thorlakshofh, Iceland to collect ships stores and exchange personnel. The second Icelandic port call was necessary because, due to fog, personnel leaving and joining the ship could not be transferred by air between Vestmannaeyjar and the mainland as originally planned.
Leg 2 began by making our way south to pick up the 20°W line at 63° 00.0' N and complete the section back to 60° 30.0' N (stations 13506-13511). At this point we crossed to Rockall (stations 13512-13520) to close off the flows to and from the north and during the last 9 days of the cruise completed three zonal sections across the Rockall Trough. The first along 57°N (stations 13521-13531) or thereabouts was a reoccupation of the Ellett line stations to continue the time series dating from 1975. The second and third, notionally along 56°N (stations 13532-13543) and 54°N (stations 13544-13553), along with the 52°N section completed earlier, where to make a detailed examination of the circulation/recirculation patterns of the water masses in the Trough.
A total of 139 full depth CTD stations were occupied during the cruise. At all stations we used the midships gantry to lower the CTD, LADCP and rosette sampler. Initially the 10 mm
CTD conducting cable was used (stations 13414-13417); however on the evening of 26 April a collapsed bearing developed in the winch and station 13418 was aborted. The wire was changed to the Deep Tow 17 min cable using a TOBI swivel and this was used until station 13436 by which time the 10 mm winch had been repaired and we changed back to this system for the remainder of the cruise.
Samples were collected at all stations for oxygen, nutrients and salts and at the majority of stations for CFC tracers/halocarbons, pigment and speciation analysis (although sometimes only from bottles corresponding to the top 200 in). In addition samples were collected at every other station for alkalinity and pH measurements and at selected stations for DON. A detailed listing of all station positions and samples collected is given in Appendix A. Continuous measurements through out the cruise included PC02 from the non toxic supply, low molecular weight atmospheric halocarbons from the foremast using a length of copper tubing and radiometric measurements of the sea surface temperature using the SISTeR instrument mounted on the foremast. Data was logged on the ship's computer system and processed using PSTAR. Navigation, meteorology, TSG VM-ADCP and ACCP was operational throughout the cruise.
2 CTD MEASUREMENTS 2.1 Equipment and operations
The equipment mounted on the CTD frame for this cruise was as follows.
• C71) Deep 04 WOCE Standard
• FSI 24 Bottle Rosette Pylon No 2.
• Chelsea Instruments Transmissometer SN 161/2642/003
• Chelsea Instruments Fluorometer SN 88/2360/108
• Simrad Altimeter 200 metre range
• RDA LADCP
• FSI 10 Litre Niskin Bottles
• SIS Digital Reversing Thermometers Nos T401,T714, T995
• SIS Digital Pressure Meters Nos P6393, P6075, P6394
During the previous cruise the FSI Rosette pylon No I had performed badly. It had failed to fire all positions whilst deployed, but would fire on deck. A replacement solenoid had been fitted in position 13 and the unit filled with silicon oil prior to the cruise. At this stage it can only be assumed that air remained inside the oil filled compartment containing the solenoids. It was decided to employ the second pylon for this cruise but this was also unsatisfactory. Whilst it would work on a short test lead, communications over the full CTD wire were poor.
The unit appeared to receive commands and fire the bottles but the return confirmation signals were corrupted. All efforts to tune the communications board failed to improve the situation. The communications board from pylon No 1 was removed, fitted in unit No 2 and tuned. The unit then performed without fault until the last 6 casts when position 7 failed to fire on a number of occasions although a confirmation signal was received.
In all 139 stations were occupied during the cruise. The 10 mm CTD cable was used with a swivel/slip ring assembly provided by RVS.
During the first test cast the oxygen sensor receptacle leaked oil continuously so this was replaced with one of a different design. This was incorrectly wired up, producing a voltage sufficiently high to affect the other DC analogue channels on the CTD. At this point power to the CTD was also lost. The fault was traced to the swivel/slip ring assembly. This was removed and the 10 mm CTD cable used without a swivel for further deployments. The wiring error was corrected but on station 13417 the sensor sensitivity was low. This was replaced and from station 13418 onwards worked satisfactorily.
Beginning with station 13419 the Deep Tow 17 mm cable was used with a TOBI swivel for the deeper stations. From station 13437 operations were resumed using the 10 min CTD cable.
SIS pressure meter SN P6075 failed on station 13440. The glass pressure housing had cracked and flooded the instrument with sea water. During heavy seas on station 13462 the frame containing SIS sensors T989 and P6132 was lost during the cast. On recovery of the package on station 13506 power and data connections to the CTD were lost. The CTD cable was short circuit at some point near the outboard end. Approximately 100 in of cable were cut off and the cable terminated.
The end caps from 3 bottles broke during the cruise and were replaced. Rob Bonner also replaced many of the taps as they became tight.
Apart from the initial problems with the FSI pylon and oxygen sensor, the rest of the equipment, both underwater and deck control units worked without fault throughout the cruise.
The cruise data were logged via the RVS level 'A' and SOC DAPS systems with few problems.
John Smithers
2.2 Data capture and processing
The CTD data were captured in dual streams: the SOC DAPS software and the RVS Level A. The main stream for processing was DAPS to PSTAR, with the RVS Level A used as backup.
DAPS
The Data Acquisition and Processing System (DAPS) utilises an Ultra-Sparc SUN workstation with an expansion box giving 16 extra serial ports, and is capable of real time acquisition/logging of data from a number of shipborne systems. The system has been developed at SOC, and is currently capable of logging CTD/SeaSoar/Bottles/GPS & Aquashuttle. On D233 it was used for logging CTD data.
For compatibility with the PEXEC suite of programs, DAPS data files are in ASCII format with time in decimal Julian day (with 1 millisecond resolution) in the first column. The variables that appear in other columns are configurable by the operator. Further compatibility with PEXEC is enabled with the use of 'dapsascin' which replaces 'pascin' and enables the user to specify a time range over which data are read in to PSTAR. Additionally, the utility 'dinfo' is a C-shell script that identifies data files logged by DAPS and displays the start and stop times of each file.
Unlike the RVS level A, B, C system where single data files for particular 'instruments' or 'data streams'remain in force for an entire cruise, DAPS allows the possibility for creating a new data file for each 'cast' or 'station' where applicable - e.g. CTD.
RVS Level A
Data are passed from the CTD deck unit the Level A. The level A averages the raw 16 Hz data to data at I Hz. Before averaging, the data are checked for pressure jumps and median despiked. The gradient of temperature over the I second sample of data is calculated. From the Level A, the data are passed to the Level B (logging) and then to Level C (archiving). Bottle firings are also logged using a separate Level A.
The Level A caused "serial overruns" when accepting and processing data from the CTD deck unit, but the clock input to the Level A was routinely removed to avoid data loss. The internal clock on the CTD Level A is sufficiently accurate over a cast if the Level A is allowed to communicate with the clock between stations.
Temperature
Temperature counts were first scaled by (2. 1) then calibrated using (2.2):
Traw = 0.0005 x Traw (2.1)
T = 0.13079 + 0.999314 x Traw (2.2)
To correct the mismatch in the temperature and conductivity measurement temperature is "sped up" by (2.3):
T +,τ dT
T= ---- (2.3)
dt
where the rate of change of temperature is determined over a one second interval and the time constant used was r = 0.25
Pressure
Raw pressure counts were scaled by (2.4) and then calibrated using (2.5):
Praw = 0. 1 x Praw (2.4)
P = -36.685 + 1.07333 x Praw (2.5)
Laboratory calibrations show the pressure sensor in DEEP04 shows little temperature dependence or pressure hysteresis, so no further corrections were made.
Conductivity
Raw conductivity was first scaled by (2.6) and then calibrated with (2.7).
Craw = 0-001 x Craw (2.6)
C = -0.015 + 0.96743 x Craw (2.7)
The offset and slope were determined using bottle samples from al-I depths of the first seven casts. Over groups of stations small offsets derived from samples deeper than 2000 dbar were added to this correction, compensating for fluctuations in the CTD and in the bottle sampling. The corrections applied to the offset are listed in Table 2. 1. After the conductivity calibration, the salinity residuals (Bottle salinity - CTD salinity) revealed no pressure dependence. Table 2.2 gives salinity residuals statistics.
Oxygen
The oxygen model of Owens and Millard (1985) was used to calibrate the oxygen data (2.8)
02 --ρ x oxysat(S,T) x (Oc-χ) x exp {α x [f x TCTD+(l -f) x Tlag]+β x P} (2.8)
where p is the slope, oxysat(S,T) is the oxygen saturation value after Weiss (1970), Oc is oxygen current, χ is the oxygen current bias, α is the temperature correction, f is the weighting of TCM (the CTD temperature) and a lagged temperature Tlag and β is the pressure correction. Five parameters, ρ, (α, β, f, χ were fitted for each station. This approach minimises the residual bottle oxygen minus CTD oxygen differences but places complete reliance on the bottle oxygen being correct. Oxygen concentrations were calculated in µmol l-1. Stations 13415- 13471 have no CTD oxygen data. Table 2.3 gives the parameters for each station and the postcalibration residual (bottle oxygen - CTD oxygen) statistics.
Transmittance, Fluorescence and Altimetry
Fluorescence was converted to voltages (2.9); this is a calibration of the voltage digitiser in the CTD. Transmittance was similarly converted to voltages with (2.
10) and further calibrated with (2.11). The altimeter had the calibration (2.12) applied.
fvolts -5.656 + 1.7267E-4 x fraw + -2.244E-12 x f2raw (2.9) trvolts -5.656 + 1.7267E-4 x trraw + -2.244E-12 x t2 raw (2.10)
trans = -0.024 + 4.81 x trvolts (2.11)
alt = -234.5 + 7.16E-3 x altraw - 0.95E-10 x altraw (2.12) Digital Reversing Temperature and Pressure Meters
Four digital reversing temperature meters were used, T401, T989, T995 and T714, and three reversing pressure meters P6075, P6394 and P6132. T401 and T714 became unfunctional after two casts (13415 and 13416), and T989 and P6132 were lost along with their frame on cast 13462. P6075 gave readings with a high offset and so was removed after cast 13439. T995 and P6394 were moved to position seven on the rosette after cast 13439 when the leaking Bottle 3 was replaced. The instruments had no calibrations applied. The arrangement of the reversing instruments is listed in Table 2.4.
Penny Holliday and Adrian New
Table 2.1 Corrections to the Conductivity Offset
Station Numbers Correction 13414 - 13415 0.0000
13416 0.0014
13417 - 13420 0.0000 13421 - 13422 -0.0010 13423 - 13424 -0.0019 13425 - 13428 -0.0027 13429 - 13436 -0.0035 13437 - 13442 -0.0044 13443 - 13456 -0.0057 13457 - 13461 -0.0043 13462 - 13474 -0.0062 13475 - 13484 -0.0067
13485 -0.0020
13486 0.0000
13487 - 13488 0.0030 13489 - 13494 0.0000 13495 - 13500 0.0030 13501 - 13504 0.0013 13505 - 13516 0.0000 13517 - 13522 -0.0038 13523 - 13546 -0.0085 13547 - 13553 -0.0070
Table 2.2 Salinity Residual Statistics
Stations Full depth Press > 2000 dbar
mean stdev n mean stdev n
13415 - 420 0.0000 0.0016 105/119 -0.0002 0.0007 35/36 13421 - 422 0.0001 0.0013 44/48 0.0000 0.0005 15/15 13423 - 424 -0.0002 0.0012 43/48 0.0000 0.0004 12/13 13425 -428 -0.0006 0.0013 82/96 -0.0001 0.0007 26/27 13429 - 436 -0.0003 0.0011 181/192 0.0000 0.0006 59/59 13437 - 442 -0.0005 0.0012 156/168 0.0000 0.0017 55/55 13443 - 456 -0.0005 0.0014 327/359 0.0000 0.0012 118/118 13457 - 461 -0.0006 0.0015 112/112 -0.0002 0.0010 29/29 13462 - 474 -0.0006 0.0012 296/306 -0.0001 0.0007 90/90 13475 - 484 -0.0004 0.0014 227/239 0.0000 0.0011 71/71
Stations Full depth Press > 1000 dbar
mean stdev n mean stdev n
13485 - 488 -0.0006 0.0018 49/64 0.0003 0.0010 20/20 13489 - 494 -0.0007 0.0016 93/102 0.0001 0.0008 37/37 13495 - 500 -0.0011 0.0018 67/79 0.0000 0.0015 13/13 13501 - 504 -0.0004 0.0012 76/80 -0.0002 0.0006 38/38 13505 - 516 -0.0011 0.0018 146/165 -0.0009 0.0013 48/50 13517 - 522 -0.0014 0.0016 62/64 0.0001 0.0028 5/5 13523 - 546 -0.0017 0.0016 323/351 -0.0003 0.0013 104/105 13547 - 553 -0.0014 0.0018 97/106 -0.0003 0.0014 40/43
Stations Full depth Press > 2000 dbar
mean stdev n mean stdev n
13415 - 553 -0.0008 0.0015 2449/2669 -0.0001 0.0008 578/585 Note: excludes residuals outside the range ± 0.005 psu
Table 2.3a Oxygen Coefficients
Station f
13419 3.8932 -0.0001856 0.03047 -0.15471 0.0000 13420 4.3942 -0.0001994 0.03201 -0.16644 0.0000 13421 4.3549 -0.0002106 0.03297 -0.17140 0.0000 13422 4.5721 -0.0002150 0.03372 -0.17447 0.0000 13423 4.0884 -0.0001978 0.02830 -0.16046 0.0432 13424 4.3194 -0.0001953 0.03061 -0.16686 0.0000 13425 4.2225 -0.0002006 0.02956 -0.16762 0.0000 13426 3.9116 -0.0002164 0.02510 -0.16735 0.5695 13427 4.0425 -0.0001972 0.02891 -0.16268 0.0000 13428 4.0133 -0.0002246 0.02568 -0.17070 0.4714 13429 4.0249 -0.0002044 0.02744 -0.16450 0.0000 13430 4.0479 -0.0001962 0.02787 -0.16263 0.0000 13431 4.0061 -0.0001899 0.02815 -0.15898 0.0031 13432 4.0062 -0.0002112 0.02650 -0.16682 0.0586 13433 4.0769 -0.0002072 0.02770 -0.16608 0.0014 13434 4.2365 -0.0002105 0.02909 -0.17056 0.0092 13435 4.1219 -0.0002040 0.02800 -0.16572 0.0000 13436 4.0620 -0.0001910 0.02888 -0.16029 0.0014 13437 4.0909 -0.0002143 0.02893 -0.16921 0.0000 13438 4.1174 -0.0001969 0.02898 -0.16194 0.0000 13439 4.1636 -0.0002015 0.02801 -0.16528 0.0000 13440 4.1699 -0.0002357 0.02719 -0.17660 0.0000 13441 4.1699 -0.0002357 0.02719 -0.17660 0.0000 13442 4.0755 -0.0001865 0.02907 -0.15843 0.0000 13443 4.1102 -0.0001937 0.02877 -0.16176 0.0000 13444 3.8414 -0.0001925 0.02518 -0.15583 0.0000 13445 4.1128 -0.0002541 0.02441 -0.18169 0.0681 13446 4.1730 -0.0002086 0.02736 -0.16823 0.0000 13447 4.1933 -0.0001994 0.02898 -0.16504 0.0000 13448 4.1099 -0.0002174 0.02683 -0.17023 0.0000 13449 4.0538 -0.0002404 0.02471 -0.17661 0.1383 13450 4.1529 -0.0001928 0.02915 -0.16101 0.0000 13451 4.1935 -0.0002424 0.02626 -0.17906 0.0105 13452 4.1438 -0.0001901 0.03086 -0.16028 0.3694 13453 4.1168 -0.0002160 0.02722 -0.16866 0.2521 13454 3.9595 -0.0002903 0.02077 -0.18933 0.2276 13455 4.0358 -0.0002717 0.02263 -0.18464 0.2081 13456 4.0621 -0.0001977 0.02685 -0.16172 0.1287 13457 3.8924 -0.0002914 0.01785 -0.18809 0.0968 13458 4.0993 -0.0001883 0.02878 -0.15783 0.0000 13459 3.8292 -0.0002274 0.02456 -0.16368 0.6503 13460 4.0668 -0.0001871 0.02939 -0.15377 0.0000
Station f
13461 4.0643 -0.0001928 0.02637 -0.15983 0.2510 13462 4.0977 -0.0001883 0.03107 -0.16010 0.4547 13463 4.1558 -0.0002025 0.02733 -0.16484 0.2028 13464 4.0706 -0.0002162 0.02739 -0.16768 0.4301 13465 4.1240 -0.0001788 0.02888 -0.15568 0.1953 13466 4.0426 -0.0001800 0.02728 -0.15447 0.2726 13467 4.0408 -0.0001607 0.03356 -0.14600 0.0653 13468 4.0369 -0.0001877 0.02775 -0.15672 0.2071 13469 4.0035 -0.0001144 0.03494 -0.12143 0.0275 13470 4.0197 -0.0001421 0.03170 -0.13586 0.0000 13471 4.0232 -0.0001466 0.03053 -0.13795 0.1418 13472 4.0857 -0.0001278 0.03401 -0.13049 0.0000 13473 4.1015 -0.0001274 0.03181 -0.12841 0.0100 13474 4.0157 -0.0001483 0.03049 -0.13802 0.1477 13475 3.9584 -0.0001510 0.03071 -0.13682 0.2261 13476 4.1159 -0.0001455 0.03258 -0.14008 0.1640 13477 4.1262 -0.0001463 0.03192 -0.13945 0.0004 13478 4.1069 -0.0001431 0.03367 -0.13837 0.0000 13479 4.0902 -0.0001093 0.03669 -0.12142 0.0000 13480 4.1136 -0.0001119 0.03776 -0.12302 0.0000 13481 4.1311 -0.0001563 0.03044 -0.14516 0.2961 13482 4.1776 -0.0001641 0.03053 -0.14996 0.0000 13483 4.0824 -0.0001313 0.03289 -0.13307 0.0000 13484 4.1391 -0.0001575 0.03208 -0.14842 0.1757 13485 4.1439 -0.0001716 0.02891 -0.15169 0.1189 13486 3.8527 -0.0001686 0.02811 -0.13986 0.1204 13487 4.0279 -0.0001777 0.02969 -0.14977 0.3983 13488 3.3404 -0.0003459 0.01240 -0.15611 0.0000 13489 3.9931 -0.0001617 0.02857 -0.14464 0.0000 13490 4.1579 -0.0001374 0.03321 -0.13760 0.0000 13491 4.3517 -0.0001181 0.03515 -0.13886 0.0000 13492 3.7728 -0.0002332 0.01913 -0.15501 0.2599 13493 4.3513 -0.0001581 0.0337 -0.15052 0.0366 13494 3.9099 -0.001948 0.02343 -0.15150 0.0458 13495 3.4606 -0.0002928 0.01306 -0.15687 0.1699 13496 4.6935 -0.0000228 0.04854 -0.11366 0.0000 13497 4.6935 -0.0000228 0.04854 -0.11366 0.0000 13497 4.7521 -0.0000478 0.04780 -0.12027 0.0000 13498 3.3632 0.0000086 0.03375 -0.04747 0.0000 13499 3.6820 -0.0003548 0.00775 -0.18266 0.2297 13500 4.5651 -0.0001527 0.03715 -0.15295 0.0000 13501 3.9879 -0.0001515 0.02919 -0.13759 0.1264 13502 3.8229 -0.0001772 0.02042 -0.14646 0.1569 13503 3.8498 -0.0001386 0.02686 -0.12758 0.0000
Station f
13504 3.2965 -0.0002900 -0.00399 -0.17312 0.3008 13505 3.4024 -0.0000449 0.03014 -0.07075 0.1081 13506 3.7764 -0.0001316 0.02749 -0.11852 0.0417 13507 3.8185 -0.0001961 0.02371 -0.15160 0.1007 13508 4.6195 -0.0000583 0.04751 -0.12374 0.0000 13509 3.8623 -0.0001574 0.02409 -0.13808 0.0000 13510 3.7361 -0.0001595 0.02336 -0.13360 0.1585 13511 3.8804 -0.0001644 0.02312 -0.14213 0.0697 13512 3.9643 -0.0001293 0.02814 -0.12476 0.0000 13513 3.8692 -0.0001898 0.02080 -0.15195 0.1830 13514 4.3931 -0.0000730 0.04343 -0.11805 0.0000 13515 3.3918 -0.0001290 0.01603 -0.12426 0.0000 13516 3.5261 -0.0001078 0.02302 -0.11126 0.0000 13517 3.5153 -0.0001203 0.02410 -0.11225 0.0000 13518 4.2182 -0.0001225 0.03380 -0.13987 0.0000 13519 3.6136 -0.0001329 0.02157 -0.13206 0.0000 13520 3.9504 -0.0001220 0.03248 -0.12166 0.0000 13521 3.1881 0.0001937 0.02806 -0.05360 0.2643 13522 3.3952 -0.0001548 0.01936 -0.12167 0.4933 13523 3.9544 -0.0000611 0.04085 -0.09102 0.0000 13524 3.8150 0.0001927 0.06369 0.07245 0.3070 13525 3.8281 -0.0000916 0.03265 -0.10744 0.0000 13526 1.7011 0.0002088 0.03409 0.39984 0.2059 13527 4.0960 -0.0000952 0.03674 -0.11837 0.0000 13528 3.7897 -0.0001508 0.02689 -0.13079 0.0000 13529 3.3870 -0.0001964 0.01727 -0.13159 0.0000 13530 2.9358 -0.0001760 0.00391 -0.12286 0.5878 13531 3.3850 0.0001800 0.09408 0.27848 0.0109 13532 3.6923 -0.0001046 0.01919 -0.14528 0.0000 13533 3.2612 -0.0002290 -0.00231 -0.18323 0.2069 13534 3.7805 -0.0000975 0.03190 -0.10884 0.0000 13535 4.1090 -0.0000711 0.03983 -0.10740 0.0000 13536 3.8213 -0.0001018 0.03201 -0.11186 0.0000 13537 3.8285 -0.0001232 0.02970 -0.12054 0.1175 13538 3.9003 -0.0001523 0.02655 -0.13830 0.0000 13539 4.0896 -0.0000817 0.03780 -0.11272 0.0000 13540 4.6298 -0.0000838 0.04377 -0.13271 0.0000 13541 3.5900 -0.0002540 0.01452 -0.16208 0.0000 13542 3.7086 -0.0001440 0.02464 -0.13129 0.0000 13543 3.3039 -0.0001773 0.01221 -0.13658 0.0000 13544 3.4017 -0.0001789 0.01748 -0.13378 0.0000 13545 5.2387 0.0001792 0.07954 -0.01202 0.0000 13546 3.7665 -0.0001538 0.02566 -0.13461 0.1568 13547 4.0605 -0.0001715 0.02809 -0.15143 0.1709
Station f
13548 3.8891 -0.0001981 0.02359 -0.15792 0.2760 13549 4.0338 -0.0001188 0.03369 -0.12926 0.0000 13550 3.7553 -0.0002015 0.02193 -0.15195 0.1625 13551 0.0114 0.0001507 0.05808 0.96337 0.6617 13552 3.2916 -0.0001415 0.01993 -0.11285 0.0000 Table 2.3b Calibrated oxygen residuals (bottle oxygen - CTD oxygen)
Stations Full depth Press > 1000 dbar
mean stdev n mean stdev n
13415 - 420 0.6684 4.9390 43/48 0.1135 1.8464 19/20 13421 - 422 -0.1281 4.2899 46/48 0.2648 1.8788 23/23 13423 - 424 -0.0345 2.7713 46/48 0.3719 1.5373 20/20 13425 - 428 0.0149 4.5643 94/94 0.4678 2.2099 41/41 13429 - 436 0.0087 2.4734 189/189 0.2762 1.8379 88/88 13437 - 442 -0.1336 2.9171 163/163 0.1313 1.7038 80/80 13443 - 456 -0.0805 2.9594 322/346 -0.0365 2.1512 160/171 13457 - 461 -0.0096 3.3870 106/106 0.1592 1.5676 50/50 13462 - 474 -0.0133 2.8616 290/290 0.1235 1.8900 134/134 13475 - 484 0.1462 3.4489 225/227 0.1406 1.2800 109/109 13485 - 500 -0.0006 3.0840 231/232 0.3436 1.6723 68/68 13501 - 520 0.0276 3.9810 309/313 0.3155 2.0037 103/103 13521 - 553 -0.0276 2.9369 475/479 0.0660 1.5887 148/148 13415 - 553 0.0063 3.1595 2449/2542 0.1937 1.9791 1021/1033 Note: excludes residuals outside the range ± 15 µmol l-1
Table 2.4 Arrangement of Reversing Temperature and Pressure Meters Stations Bottle Instrument
13415 1 T401, T989, P6075
3 T995, P6394
13416 1 T401, T989, P6075
3 T995, P6394
11 T714, P6132
13417 - 13439 1 T989, P6075
3 T995, P6394
11 P6132
13440 - 13462 1 T989, P6132
7 T995, P6394
13463 - 7 T995, P6394
2.3 Post-cruise laboratory calibration
The calibration data used during D233 were from laboratory calibrations in July 1996. The post cruise calibrations carried out in October 1998 produced the following data:
scale offset linear
Pressure 0.1 -38.3 1.0738
Temperature 0.0005 0.13121 0.99928
The difference in pressure due to the linear term is only 2.6 dbar at full scale. The difference in temperature due to the offset is only 0.42 m°C and the linear terms differ by 1.65 m°C at full scale. It was concluded that these differences were sufficiently small that no additional calibrations need be applied.
S. Cunningham 2.4 References
Owens, W. B. and R.C.Millard, 1985: A new algorithm for CTD oxygen calibration. J. Phys. Oceanogr., 15 621-631
Weiss, R. F., 1970: The solubility of nitrogen, oxygen and argon in water and seawater.
Deep-Sea Res. 17 721-735.
3 LOWERED ADCP MEASUREMENTS
The Lowered Acoustic Doppler Profiler (LADCP) is an RDI 150 kHz BroadBand ADCP (phase 111) with 30 degree beam angles. It is mounted vertically within the CTD frame with the bottom of the transducers protected by the base of the frame. The LADCP was installed on the CTD frame at the beginning of the cruise. It had been hoped to use a rechargeable power pack to avoid the regular removal and replacement of batteries. Unfortunately the enclosing pressure case for the rechargeable system could only be used down to 1000 db. Ten alkaline battery packs were on board at the start of the cruise including two part used ones. Two further packs were brought out by personnel joining the ship from Iceland for the last week of the cruise. To change the batteries, the pressure case was either removed from the frame and batteries removed in the lab, or in quiet sea states where no risk of spray was present, the case was left on the frame and the batteries removed. The latter method impeded sampling on one occasion because of the danger of wetting exposed cables. These slight difficulties will be avoided in future by use of the rechargeable system.
A few minutes before each cast, a command file was downloaded to the unit from a PC in the deck lab via a serial link. On this cruise the same command file was
used throughout. A listing is given in Appendix C. It was decided at the beginning of the cruise to use bottom tracking throughout. This reduces the number of water track pings, but is justified because it allows a second independent estimate of the bottom current to be made. At regular intervals the instrument emits a bottom ping to test for range. Once the bottom was found the instrument recorded the velocity of the ground with respect to the package. It was hoped that this would provide a check of the quality of the absolute velocity data calculated by the more round about route described below.
The data were recorded internally and downloaded at the end of each cast by connecting a data link to the package from the PC. RDI utilities BBTALK and BBSC were used to interrogate the profiler and to download data to the PC.
Power is supplied to the profiler via the serial cable in order to conserve the battery pack.
Data were transferred to the UNIX workstations via PC-NFS and then processed using a combination of PERL scripts and MATLAB m-files deve-loped by Eric Firing at the University of Hawaii. Processing was done in a number of steps which are briefly described:
i. The binary data were first scanned to find useful information from the cast such as time at the surface, time at the bottom and number of ensembles.
ii. The data were then read into a CODAS database. Magnetic variation and position were added to the database at this stage.
iii. When CTD data were available, the pressure temperature and salinity data were added to the database in order to correct for the variation of sound speed with depth.
iv. Absolute velocities were then found by calculating horizontal velocity shear to eliminate package motion, integrating with time to calculate the barotropic term and then merging with navigation data to remove the motion of the ship.
Bottom velocity data are not included in this processing path and had to be extracted manually from the binary file on the PC and processed separately from the water track data. Preliminary comparisons were made between the resulting velocities and the near bottom velocities extracted from the absolute water track data. No clear interpretation was achieved and more work is required here.
Comparisons with geostrophic profiles from the CTD data for the 20°W line are shown in Figure3.1. A horizontal line is drawn on each plot at 3210 m which is the level at which the
Figure 3.1 Comparison of LADCP data with geostrophic profiles calculated from CTD data
geostrophic velocity is assumed to be zero. If the water depth is shallower than 3210 m, a zero velocity is assumed at the bottom.
Steve Alderson, Lisa Redborn and Chris Wilson 4 VESSEL MOUNTED ADCP MEASUREMENTS
4.1 Description and Processing
The instrument used was an RDI 150 kHz unit, hull-mounted approximately 2 m to port of the keel of the ship and 33 in aft of the bow at the waterline. Data Acquisition Software (DAS), version 2.48, was run on a PC to acquire the data.
With the exception of a few interruptions (see Problems section below) the instrument operated continuously from JD 114 to JD 15 1 in the water-tracking mode, and set to use 3 beam solutions for determining velocities as beam 3 (the forward beam) was not working. Ping data were averaged by the DAS into 2 minute ensembles, and 64 x 8 m depth bins were used for the entire cruise with a depth offset of 13 m included in the processing to allow for the ship's draught and the 'blank after transmit' period.
4.2 Daily Processing
• Acquisition of ADCP water-tracking velocities from Level C RVS files and conversion to PSTAR format using the PSTAR program adpexecO.
• Correction of the times of each ADCP ensemble to account for the linear, 18 second per 24 hour drift of the PC clock using program adpexec 1.
• Correction of ADCP heading data (which the DAS reads from the ship's gyrocompass) using the Ashtech minus gyro heading differences (program adpexec2).
• Calibration of the shear profiles, taking account of errors in signal amplitude and transducer alignment using a working calibration determined by a 'zig-zag' run in water-tracking mode at the end of cruise D232 (see Calibration section below). This was done with program adpexec3.
• Merging velocity profiles with navigation fixes obtained from the GPS4000 navigation files to effectively remove the ship's speed from the ADCP velocities, thus giving absolute velocities (program adpexec4).
• Separation of each day's ADCP data into 'on station' and 'underway' files. Each on station file corresponded to a CTD station and the velocities in these files were plotted as vectors, averaged over the period of time the ship held station and plotted against LADCP velocity profiles for comparison.
4.3. Calibration
The ADCP is calibrated to take account of the orientation of the transducer mounted in the hull (the transducer orientation is intended to be fore-aft). Ideally, the ADCP's bottom-tracking mode is employed in shallow (<500 m) water to determine the amplitude factor, A, and the alignment angle error, 0. However, the absence of beam 3 meant that the bottom-tracking mode of ADCP operation was unavailable throughout this cruise. Instead, a total of three zigzag runs in regions known to have fairly uniform currents were used to calibrate the ADCP; one conducted at the end of cruise D232 and the other 2 conducted on JDs 137 and 151 of cruise D233.
During each zig-zag run, the Bridge were asked to make an initial turn of 44°
(either to port or starboard as preferred) away from the base course at between 10 to 15 seconds past the hour. The new heading was maintained at a steady speed of 10 knots for 20 minutes. At 20 minutes past the hour, a 90° turn back towards the base course was made, and thereafter alternate 90° turns were completed, with 20 minutes steaming between each turn. As far as possible, the
same speed through the water was maintained throughout, and the entire calibration run lasted for about 3 hours in each case.
Data from the zig-zag runs were processed as described above but with A set to 1 and Ø set to 0° in adpexec3. Data recorded during the ship's turns were discarded, and the components of ship's velocity and ADCP velocity (i.e. 'water past the ship') were each averaged together for each of the 'zigs' or 'zags' between turns. The differences in each of these four averaged components were then calculated for before and after each turn such that:
adpe = difference between averaged east-west component of ADCP velocity before and after turn.
adpn = difference between averaged north-south component of ADCP velocity before and after turn.
ve = difference between averaged east-west component of ship's velocity before and after turn.
vn = difference between averaged north-south component of ship's velocity before and after turn.
So, a zig-zag run comprising eight 90° turns produces eight different values of each of these 4 quantities.
Equations 4.1 and 4.2 are then used to find Ø and A:
(ve x adpn) - (vn x adpe)
tan Ø = --- (4.1) (vn x adpn) + (ve x adpe)
(vn x adpn) + (ve x adpe) (4.2)
A = - --- cos Ø x (adpe2 + adpn 2)
The 8 values of Ø and A were then averaged to give the best estimate of the true amplitude factor and transducer misalignment angle. The calculations were made using data from several bin depths to further reduce the likelihood of errors.
The zig-zag calibration at the end of D232 gave average values of Ø and A as 2.64° (with an sd of 0.0l°) and 0.9917 respectively and these values were used in adpexec3 to calibrate all data from this cruise. Data from the zig-zag run on JD 137 of this cruise have not been worked up due to the poor quality of ADCP data acquired on that day (see Problems below). The final run, conducted on the last day of the cruise (JD 151) will be worked up ashore.
4.4 Problems
Gaps in the otherwise continuous ADCP data set were as follows:
• JD 120, 19:17 to JD 121 04:45: ADCP was still working but logging to level C had stopped.
• JD 122, 17:20 to 17:30: ADCP was interrupted to retrieve missing data for the previous day from raw files on the P.C.'s hard disk.
• JD 127, 08:35 to 09:35: Power cut.
• JD 139, 04:37 to 04:40 ADCP interrupted to change settings in DAS.
• JD 140, 12:00 to 12:40: ADCP interrupted to change settings in DAS.
• JD 141, 07:30 to 08:42: ADCP stopped for testing.
• JD 144, 11:45 to 11:55: ADCP stopped to check settings in DAS.
The majority of these interruptions were necessary as a result of a persistent problem occurring with the DAS software prior to the data being logged in the RVS files.
As beam 3 was known not to be working, the DAS software was set to calculate 3 beam velocity solutions from the start of the cruise. However, it appeared that the DAS software was still using 4 beam solutions at certain times, such that 'bad' data from beam 3 were included which subsequently degraded the calculated velocities. The use of 4 beam solutions was identified in the ADCP data files by the presence of non-zero error velocities (error velocities being only determinable when all 4 beams are used). Throughout the cruise, the occasional non-zero error velocity in the data files occurred, but during the period from JD 133 to JD 145, the percentage of 4 beam solutions being used was large enough to produce many spurious velocities which considerably degraded the data set.
JD 137 was perhaps the worst day in terms of poor data quality during this period.
4 beam solutions were particularly prevalent at depth and in the 'underway' data between CTD stations. On station velocity profiles were still reliable to about 150 in depth, as confirmed by comparisons with LADCP data. At depths greater than 150 in, large changes in current velocity (up to 60 cm s-1) appeared to occur simultaneously throughout the water column whenever the ship's speed changed, which was clearly erroneous.
The abundance of 4 beam solutions at depth may indicate that whenever the DAS software receives back-scattered signals which it considers to be too low, it listens to all 4 beams in an attempt to improve the signal to noise ratio, and subsequently calculates velocity using ping data from all 4 beams. However, this problem will require further investigation ashore.
Lisa Redbourne, Steve Alderson, Harry Bryden , Dave Jolley
5 NAVIGATION DATA 5.1 Differential GPS4000
Differential GPS4000 navigation data (ship position, heading, speed over ground, satellite fix parameters) were acquired every second throughout the cruise, giving the ship's position to within 5 in.
Daily Processing:
• Acquisition of GPS4000 data from RVS files using the gpsexec0 program.
• Quality control of data in which data are deleted wherever poor positioning accuracy is indicated by satellite fix parameters.
• Averaging ship velocity data into 2 minute bins for subsequent merging with ADCP data.
Data Quality
The percentage of 'good' data acquired during a 24 hour period ranged from a minimum of 97.5% on JD 108 to a maximum of 99.8% on JD 103.
5.2 Ship's Gyrocompass
Two SG Brown gyrocompass units are installed on the bridge. Ship heading was logged every second via a level A microprocessor.
Daily Processing
• Acquisition of gyro heading data from RVS files using the gyroexecO program.
Data Quality
The percentage of 'good' data acquired during a 24 hour period ranged from a minimum of 99. 1 % on JD 96 to a maximum of over 99.9% on JD 99.
5.3 Ashtech 3DF GPS Attitude Determination
The Ashtech 3DF GPS is a system comprising four satellite receiving antennae mounted on the boat deck and the roof of the bridge with a receiver unit in the bridge itself. Every second the Ashtech measures ship attitude (heading, pitch, roll) and these data are used in post-processing to correct ADCP current measurements for 'heading error'. This post-processing is necessary as the ADCP uses the less accurate but more continuous ship's gyro headings to resolve east and north components of current. With each attitude acquired are measures of the maximum measurement rms error and maximum baseline rms error which permit poorly determined attitudes to be flagged during processing.
Daily Processing:
• Acquisition of Ashtech data from RVS files using the ashexec0 program.
• Quality control of Ashtech data using ashexec 1.
• Averaging into 2 minute bins to be compatible with ADCP data and determination of the 'aghdg' parameter (the correction applied to gyro headings) using ashexec2.
• Plotting daily time series of a-ghdg and manually editing out any remaining outliers from the data using PLXYED and ashexec3.
Data Quality
The percentage of 'good' data acquired during a 24 hour period ranged from a minimum of 87.7% on JD 107 to a maximum of 98.3% on JD 103. Manual editing was required on JD's 103, 104, 105, 107 and 108 with a maximum of 9, 2 minute averaged data cycles being removed on JD 107. The largest gap in a-ghdg data was also on JD 107 and lasted approximately 2 hours.
5.4 Problems with Navigation Data
A power cut of approximately 30 minutes from 08:35 to 09:05 on JD 127 caused all navigation instruments to stop logging during that period. On the same day, the Ashtech stopped logging for 4 hours from 05:40 to 09:40, a loss of data initially unrelated to the power cut. It also stopped logging for 10 minutes at about 20:00 on JD 138.
Lisa Redbourne and Steve Alderson
6 METEOROLOGICAL MEASUREMENTS 6.1 Aims
The primary goals of the surface meteorological and radiative flux measurements made on D233 were:
i. To evaluate sources of error in the downwelling longwave flux as measured by an Eppley pyrgeometer using a circuit that allows the temperatures of various components of the pyrgeometer to be recorded.
ii. To investigate the dependence of the corrected downwelling longwave on the amount of cloud cover, the air temperature and the humidity, and to develop a new parameterisation for this flux if existing ones prove insufficient. The large range of latitude, 20 - 63.5 °N, covered during the cruise provided an ideal opportunity for this investigation to be carried out under a wide variety of atmospheric conditions.
In addition, measurements of various meteorological variables were made with a range of sensors as standard. Finally, estimates of the surface wind stress were obtained by the inertial dissipation method using high frequency wind speed measurements made with a Research R2A sonic anemometer.
6.2 Sensors Deployed Meteorological variables
Measurements from a combination of sensors mounted for this cruise alone and the standard RVS sensor suite were made during the cruise. Information from a total of 19 sensors giving the wind speed and direction, air and sea surface temperatures, atmospheric humidity and pressure, downwelling radiative fluxes and various component temperatures for one of the pyrgeometers was logged using the GrhoMet instrumentation system, details of these sensors are given in Table 6.1 and their deployment positions are shown schematically on Figure 6. 1.
Prior to D232, modifications were made to the RVS sensor system such that data from the standard meteorological sensors is now recorded via the RVS Surfmet system which outputs data every 30 sec to the Level 'B' and every 5 sec via an RS232 serial link to the GrhoMet PC. GrhoMet separately acquires data from the cruise specific sensors at a 5 sec sampling rate from a new Rhopoint box mounted on the starboard rail of the foremast platform. The two data streams are merged by the GrhoMet PC every 5 sec and written to the level B.
Cloud observations
Observations of the total cloud amount and of the type and amount of low, medium and high altitude cloud were made at hourly intervals during daylight and typically three hourly intervals at night throughout the cruise using the Met. Office 'Cloud Types for Observers' guide as a reference. Over 600 observations were made in total covering a wide variety of cloud
Table 6.1 Variables and sensors logged by the GrhoMet system.
Variable Position Instrument Note
Wet and Dry Bulb [psy 1dry psy 1wet]
STBD side of foremast platform (aft sensor)
Psychrometer HS 1019 (SOC)
(1) Wet and Dry Bulb
[psy 2dry psy 2wet]
STBD side of foremast platform (fwd sensor)
Psychrometer IO2003 (SOC)
Air temp [airtemp 1] STBD side of foremast platform
Vector Inst. 203/16924 Air temp [airtemp 2] PORT side of foremast
platform
Vaisala HMP44L S5040001 (RVS) Longwave [1wave2] Top of foremast (port
sensor)
Eppley PIR 27960 (SOC)
Pyrgeometer thermopile voltage [e]
Top of foremast (starboard sensor)
Eppley PIR 31170 (SOC)
Pyrgeometer dome temperature [td]
Top of foremast (starboard sensor)
Eppley PIR 31170 (SOC)
Pyrgeometer body temperature [ts]
Top of foremast (starboard sensor)
Eppley PIR 31170 (SOC)
Shortwave [swavep] Gimbal mounted on port side of foremast platform
Kipp & Zonen CM6B 962276 (RVS) Shortwave [swaves] Gimbal mounted stbd
side of foremast platform
Kipp & Zonen CM6B 962301 (RVS) Wind Speed & Directions
[wspeed1 wdir 1]
PORT side of foremast platform on vertical pole
Windmaster sonic No. 126 (SOC) Wind Speed [wspeed2] PORT side of foremast
platform on horizontal pole projecting forward
Vaisala (RVS) (2)
Wind Direction [wdir2] PORT side of foremast platform on horizontal pole projecting forward
Vaisala (RVS) (2)
SST [sst] Trailing from 6 m scaffold pole off port Bow
Trailing Thermistor SOAP pdm004/53 (SOC)
Pressure [press] Lab Vaisala PTB 100a
R0450005 (RVS)
The variable names in the data files are shown [thus]. (RVS) indicates that the sensor is part of the standard ship's system; (SOC) that the instrument was added for the cruise.
Notes (1). Dry bulb reading noisy for significant fraction of cruise (2). Wind speed persistently biased low by 10 - 15%
Figure 6.1. Plan view of the bow of the ship showing meteorological sensor positions for Discovery Cruise D233.
conditions, the resulting dataset will be used for an investigation of how the downwelling longwave flux may best be parameterised in terms of the cloud cover and other variables.
Wind stress
High frequency wind speed measurements were made with a Gill Instruments Solent Sonic Anemometer ( R2 Asymmetric Model, serial no. 37) which was mounted on the starboard side of the foremast platform. The anemometer was operated in Mode 1 and the 21 Hz sampled data were logged using a PC system situated in the Plot that was also used for the GrhoMet output. Two programs were used to sample the data, standard sonic and Gill sonic the latter being a new program which provides additional parameters during the data processing cycle. In each case wind speed spectra and spectral levels are determined from the raw data. Standard sonic has a 10 minute sampling period starting each quarter hour. It derives a single PSD value in the range 2 - 4 Hz from the average of 12 data sections, each of which contains 1024 data points, taken within the sampling period. Gill sonic calculates PSD values in the sub-ranges 2 - 3 Hz, 3 - 4 Hz, 4 - 5 Hz and 5 - 6 Hz from each section. It was initially run with an 8 minute sampling period which was increased to 10 minutes on JD 142 to allow comparisons to be made with the standard sonic program.
6.3 Sensor Performance Air temperature and humidity
Dry bulb air temperature measurements were obtained from the two psychrometers, the vector sensor and the RVS humidity sensor. An initial problem with the direction of the fan supply to the psychrometers led to them being biased high relative to the vector sensor by of order 0.5 °C for the first four days of the cruise. The fan supply was reversed on JD 118 and agreement between the three sensors to typically within 0. 1 °C was subsequently obtained with the exceptions due to noise noted below. The signal from the aft dry bulb sensor became increasingly noisy from JD 132 to 135, at which point the foremast connections were checked and sprayed with moisture repellent. No obvious problems were found but following the checks no further noise problems occurred until JD 138 and intermittently thereafter. The RVS sensor had an offset of about -0.4 °C with respect to the vector and the psychrometer values after the fan problem was corrected. Regarding the wet bulb temperatures, the aft psychrometer showed a positive bias, increasing with the amount of downwelling shortwave, of up to 0.2 °C with respect to the foreward sensor. Its cause remains uncertain ; there was no obvious effect of shortwave on the relative values of the psychrometer dry bulb measurements. Given the noise in the aft dry bulb values and the apparent bias in the wet bulb we suggest that the foreward sensor values be used in any subsequent analysis.
Radiative Fluxes-Longwave
Measurements of the downwelling longwave flux were obtained with two Eppley pyrgeometers mounted on top of the foremast. The first radiometer, No 27960, was operated in standard mode with output according to the manufacturers calibration; the second, No 31170, was fitted with a circuit, supplied by Dave Hosom of Woods Hole Oceanographic Institute, which allowed the dome and sensor temperatures and the thermopile voltage to be recorded. Given these parameters and pre-cruise laboratory calibrations for the effect of the dome- sensor temperature difference and shortwave leakage on the measured longwave flux a corrected longwave field was produced for 31170. The dome- sensor temperature difference was found to be a function of both the incident shortwave and the relative wind speed, typical values being 1.8 °K and 1.2 °K for a shortwave flux Of 1000 Wm-2 and relative wind speeds of 3 and 10 ms-1 respectively. The magnitude of the required correction to the measured longwave for this effect was of order 10 Wm-2. In post-cruise analysis we plan to develop an empirical correction for the dome-sensor temperature difference and assess the level of agreement of the longwave flux measured by the two instruments once it has been applied to 27960. A paper is being prepared on the results of the longwave study (Pascal and Josey, 1998).
Radiative Fluxes -Shortwave
Measurements of the shortwave flux were obtained with two RVS solarimeters mounted on the port and starboard sides of the foremast platform. Shadowing proved a problem as on previous cruises but in periods when the two sensors were clearly illuminated they were in good agreement e.g. to within 5 Wm-2 for a downwelling flux of order 800 Wm-2 on JD 122. At night, both sensors were typically within 2 Wm-2 of zero.
Wind Speed and Wind Stress
Wind speed measurements were made with two Solent Sonic anemometers (one Research R2A; one Windmaster) and an RVS cup anemometer. The RVS sensor was mounted on a pole projecting forward from the port side of the foremast platform with several other instruments in close proximity. The wind speeds that it recorded were typically biased low by 10- 15 % relative to those measured by the Windmaster Sonic. The Windmaster and Research Sonics agreed in the mean to within 0.2 in s-1 giving mean wind speeds of 8.03 and 8.20 in s-1 respectively.
Output from the two sonic programs was compared for the period JD 1421445 - 1460915. Relative to the standard sonic output, Gill sonic gave mean wind speed values that were typically lower by 0.8% and PSD values higher by 5%.
Generally both systems agreed well although the standard system was more prone to wayward data points particularly at low wind speeds; this being primarily due to vibration peaks in the spectrum affecting data in the 4 Hz region.
Preliminary determinations of the wind stress were carried out with the standard sonic output using only data obtained within 30° of the bow in order to avoid biases in the measured speeds arising from flow distortion by the ship. The following least squares fit (equation 6. 1) to the variation of the neutral drag coefficient with 10 in wind speed was found for wind speeds > 6 m s-1,
103 Cdn = 0. 64 + 0.045u10n (6.1) Sea Surface Temperature (SST).
Measurements of the bulk sea surface temperature were made with a trailing thermistor (soap) and the thermosalinograph (TSG). In addition the skin temperature was measured with the Scanning Infrared SST Radiometer (SISTeR) deployed by Tom Sheasby from the University of Leicester. The SISTeR measurements are covered in a separate section ; comparisons of all three sensors carried out as part of the SISTeR study showed that there was an intermittent slowly decaying offset in the SST as measured by the TSG remote sensor. The sensor was replaced on JD 134 at 1440 but the offset problem continued for the remainder of the cruise.
6.4. Summary of Measurements
A wide variety of atmospheric conditions were sampled as the cruise track took the ship from the tropics to the sub-Arctic. Summary time series for each of the measured variables are shown on Figure 6.2. During the first two weeks of the cruise, Trade wind conditions dominated with a steady flow of relatively warm, dry air from the North-East at typical speeds of 7-11 m s-1. The winds slackened and veered to the south-west on JD 127 with the ship at latitude
38 °N. Several calm days followed, prior to the strongest winds of the cruise, in the range 18-20 m s-1 on JD 130, caused by a low pressure system to the west of Portugal. A high pressure system centred on the UK ensured relatively calm conditions for the remainder of the leg to Iceland with increasing cloud cover and relative humidities. Along the section from Cape Verde to Iceland the air temperature dropped from 22 °C to 7.5 °C and the specific humidity ranged from 13.5 gkg-1 to 5.5 gkg-1. The winds increased again and shifted to the Northeast during the Rockall Section and survey of the Rockall Trough as a low pressure system developed over Scandinavia.
6.5 References
Pascal, R. W. and S. A. Josey (1998). Accurate radiometric measurement of the atmospheric longwave flux at the sea surface, J. Atmos. Oceanic Technol., in preparation.
Simon Josey and Robin Pascal
Figure 6.2. Summary plot of meteorological conditions experienced during the cruise. Time series show three hourly mean values of wind speed (speed), wind direction - from - (dirn), sea surface temperature (sst), atmospheric pressure (press), dry bulb temperature (psy2dry), specific humidity (QAIR), relative humidity (RH), downwelling longwave (Iwave1c) and downwelling shortwave (maxswvps).
7 SALINITY MEASUREMENTS 7.1 Salinity sampling
Salinity samples were drawn from each Niskin bottle plus a duplicate each from Niskins 1 and 5. The only exception to this was on the very shallow casts, where only one duplicate was taken (from bottle 1). Sample bottles used were the standard 200 ml glass bottles with disposable plastic inserts and screw caps.
7.2 Measurement
Two salinometers were installed in the constant temperature laboratory, (Guildline models 8400 and 8400A), but only the 8400A unit was used; the 8400 being carried as a backup. The salinometer tank temperature was set at 21 °C but to maintain a laboratory temperature of 20 °C degrees, the laboratory air conditioning unit was set at 19 °C.
The salinometer was standardised at the start of each crate of samples using batch P133 standard seawater (production date Nov '97) and salinity was calculated from the Guildline ratio using Microsoft Excel spreadsheet macros.
Only 2 ampoules were found to be high in salinity, which was probably caused by imperfect sealing, allowing evaporation. Comparison of the CTD/bottle sample salinities showed that differences were better than 0.001.
The salinometer generally performed very well, except on 2 separate occasions when a reading was being taken the ratio display started counting down at approximately 0.00001 per second, and had dropped 0.00250 after 4 minutes.
After the cell had been flushed and refilled, the display behaved normally and gave expected readings. No obvious reason could be found for this.
The only other problem encountered, was that the outlet tubing on the peristaltic pump came off on 2 occasions. In the first instance it was pushed back on to the outlet nipple, but it came off again after about 12 hours. This time the pump unit was replaced with the one fitted to the other salinometer. However this unit suddenly made the salinity readings go high. It was then noticed that the pump outlet tubing on this unit had cable ties fitted for extra security, but were missing from the original unit. The original pump was fined back on the salinometer with cable ties securing the tubes. The salinity readings then returned to normal and no further problems were experienced. It is suspected that some salt crystals may have been trapped in the spare pump, causing the salinity to go high when flushed through the cell.
Rob Boner, Steve Alderman, Dave Jolly, Chris Wilson
